In recent years, from the viewpoint of energy problems and environmental issues, there have been developed artificial photosynthesis technologies to electrochemically convert the solar light into a chemical substance while imitating photosynthesis of plants. In these technologies, when the solar light is converted into a chemical substance and stored in a cylinder or tank, they have the advantages that the energy storage cost is lower and the loss caused by storage is smaller than those when the solar light is converted into electricity and the electricity is stored in a storage battery.
There has been being established a technique for taking hydrogen as a chemical substance (principally chemical fuel) from water. As an example using optical energy, there has been considered a photoelectrochemical reaction device using a laminate (such as a silicon solar cell) in which a photovoltaic layer is held between a pair of electrodes. In an electrode on a light irradiation side, water (2H2O) is oxidized by light energy to obtain oxygen (O2) and hydrogen ions (4H+). In the electrode on the opposite side, hydrogen (2H2) or the like as a chemical substance is obtained using the hydrogen ions (4H+) produced at the electrode on the light irradiation side and a potential (e−) generated in the photovoltaic layer. There has been also known a photoelectrochemical reaction device in which silicon solar cells are stacked (see, for example, (S. Y. Reece, et al., Science vol 334.pp.645(2011)). However, in those methods, although the conversion efficiency from solar light to chemical energy is high, it is inconvenient to store and transport produced hydrogen. In consideration of energy problems and environmental issues, it is preferable that solar light is converted into not hydrogen but a carbon compound which is easily stored and transported.
Meanwhile, a technique for highly efficiently converting abundant CO2 into a chemical substance or the like useful as chemical fuel has not yet been established. As a photoelectrochemical reaction device using light energy, although it is a technique on a laboratory level, there has been known a system of a two-electrode system in which an electrode having a reduction catalyst reducing carbon dioxide (CO2), for example, and an electrode having an oxidation catalyst oxidizing water (H2O) are provided, and these electrodes are immersed in water dissolved with CO2. In the electrode having the oxidation catalyst, as in the case where hydrogen is taken from water, H2O is oxidized by light energy to obtain oxygen (½O2), and, at the same time, obtain a potential. In the electrode having the reduction catalyst, a potential is obtained from an electrode inducing oxidation reaction, whereby CO2 is reduced to produce formic acid (HCOOH) or the like (see, for example, S. Sato, et al., Journal of the American Chemical Society vol. 133.pp.15240(2011)). As other CO2 reduction electrodes which have been reported so far, the electrodes described in JP 2011-094194A, Y. Chen, et al., Journal of the American Chemical Society vol. 134.pp.19969(2012) and W. Zhu, et al., Journal of the American Chemical Society vol. 135.pp.16833(2013), etc. are exemplified.